1. Field of the Invention
The present invention relates to a touch panel of an input device used in a computer and the like, and particularly relates to a 5-wire touch panel.
2. Description of the Related Art
Prevalent touch panels include resistive types, surface acoustic wave types, capacitive types, infrared types, and the like. Further, resistive types include those termed a 4-wire type and a 5-wire type. For example, a 5-wire touch panel is disclosed in Japanese Laid-Open Patent Publication No. 04-137026.
In a 5-wire touch panel shown in FIG. 16, transparent rectangular resistive films 14 are laminated on opposing surfaces of two substrates 12a and 12b, respectively. Although not shown in the figures, spacers are provided so that both substrates 12a and 12b oppose each other at a particular distance. Electrodes 56a, 56b, 56c, and 56d are formed on the four sides of either one of the resistive films 14 (FIG. 17). The substrate on which the electrodes 56a, 56b, 56c, and 56d are formed is the substrate 12a, and the substrate opposing the substrate 12a is the substrate 12b. 
FIG. 17 shows the substrate 12a and the peripheral circuitry thereof. A positive voltage is applied to both ends of the first electrode 56a and a negative voltage is applied to both ends of the second electrode 56b, so that a potential gradient is formed between the first electrode 56a and the second electrode 56b. Then, after switching, voltages are applied to the third electrode 56c and the fourth electrode 56d, similarly to the above, so that a potential gradient is formed therebetween. When these two potential gradients are repeatedly switched in a short time by switches S1, S2, S3, and S4, the potential gradients are alternately formed horizontally and vertically on the substrate 12a. When the flexible substrate 12b is touched, the resistive films 14 make contact with each other, and therefore the substrate 12b can measure the potential formed on the substrate 12a. The location of the touch is obtained by detecting the potential at the time of the touch.
In order to prevent leakage current from flowing through the electrodes 56a, 56b, 56c, and 56d, normally, the electrodes 56a, 56b, 56c, and 56dare each provided with a moderate resistance. Due to this, the equipotential lines distort near each electrode. Conventionally, in order to prevent the distortion, the shape of an electrode that is less likely to cause distortion is sought and determined by trial and error. For example, as in Japanese Laid-Open Patent Publication No. 04-137026, it is proposed that electrodes 58 be T-shaped (FIG. 18).
However, it is difficult to design the T-shaped electrodes 58 disclosed in Japanese Laid-Open Patent Publication No. 04-137026, due to variations in size of the T-shape, and the like. Particularly, the determination of the shapes of the electrodes 58 by trial and error requires a great deal of effort. In addition, the electrodes 58 that protrude the furthest inward to the resistive film 14 limit the usable area of the resistive film 14.